Glyoxalated polyacrylamide (G-PAM) can be used in a variety of paper grades to provide paper with dry and temporary wet strength. For example, glyoxalated polyacrylamide can increase the initial wet strength of many household tissues which come in contact with water in use. Glyoxalated polyacrylamide can also be applied to increase the compression strength and the dimensional stability of many board-grade paper products.
Glyoxalated polyacrylamide has been prepared by reacting glyoxal with a cationic polyacrylamide in slightly alkaline aqueous solution and stabilized under acidic conditions. Glyoxalated polyacrylamides typically contain relatively low amounts of cationic monomer (about 5 mole percent) to limit the cationic charge contribution of this component.
U.S. Pat. No. 8,435,382 relates to storage-stable glyoxalated polyacrylamide polymers. The glyoxalated polyacrylamide polymer comprises from about 75% to about 10% acrylamide and from about 25% to about 90%, by weight cationic monomer, wherein the cationic monomer may include diallyldimethylammonium chloride monomer. These glyoxalated polyacrylamide polymers are used in an amount of at least 10% by weight to improve storage stability.
U.S. Patent Application Publication No. 2011/0056640 relates to resins useful for imparting strength to paper, the process of incorporating these resins into paper, and paper produced containing the resins. A reactive cationic resin comprising a copolymer produced from a comonomer which is dialdehyde reactive (acrylamide), a cationic comonomer (DADMAC), and a dialdehyde (glyoxal), wherein the cationic comonomer comprises greater than 10 mole % of the copolymer before reaction with dialdehyde.
Conventional G-PAMs have molecular weights of 100,000 Daltons or less to avoid gelation during the glyoxalation process. Also, increasing the ratio of DADMAC monomer to acrylamide monomer in G-PAMs increases the effective rate of pulp dewatering and allows for G-PAMs to be produced at a higher total solids concentration. However, increasing the ratio of DADMAC to acrylamide is expected to reduce strength efficiency, because the dry strength efficiency of G-PAMs is generally believed to derive from covalent bonds which form between the pendant aldehyde groups associated with glyoxal bound to amide groups from the acrylamide portion of the polymer. Due to this tradeoff, G-PAMs are conventionally made from acrylamide/DADMAC copolymers with a molecular weight in the range of 5,000 to 15,000 Daltons, and a weight ratio of acrylamide/DADMAC with a weight of acrylamide/DADMAC of 90-95 wt. % acrylamide to 10-5 wt. % DADMAC.
A need remains for a cellulose reactive glyoxalated copolymer that increases both the dewatering rate of a treated cellulosic slurry and also the dry strength of the paper/board made from the slurry.